This invention is generally directed to an improved overcoated photoresponsive device, and more specifically to an improved photoresponsive device where the sensitivity thereof can be varied or enhanced allowing such a device to be capable of being responsive to visible light, and infra-red illumination needed for laser printing. In one important embodiment of the present invention, there is included in the device situated between a photogenerating layer and a hole transport layer, a composition such as a photoconductive composition, which composition is primarily responsible for enhancing or reducing the intrinsic properties of the photogenerating layer in the infra-red and/or visible region of the spectrum thereby allowing such device to be sensitive to either visible light and/or infra-red wavelengths.
The formation and development of electrostatic latent images on the imaging surfaces of photoconductive materials by electrostatic means is well known, one such method involving the formation of an electrostatic latent image on the surface of a photosensitive plate, referred to in the art as a photoreceptor. A photoreceptor can be comprised of a conductive substrate containing on its surface a layer of photoconductive insulating material, and in many instances, there is incorporated therein a thin barrier layer situated between the substrate and the photoconductive layer, which layer prevents charge injection into the photoconductive layer upon charging of its surface. Charge injection in these devices adversely affects the quality of the resulting images.
Numerous different photoconductive members for use in xerography are known including for example, a homogeneous layer of a single material such as vitreous selenium, or composite layered devices, containing a photoconductive substance, dispersed in other substances. An example of one type of composite photoconductive layer used in xerography is described for example, in U.S. Pat. No. 3,121,006, wherein there is disclosed a number of layers comprising finely divided particles of a photoconductive inorganic compound dispersed in an electrically insulating organic resin binder. In a commercial form, there is coated coated on a paper backing the binder layer containing particles of zinc oxide uniformly dispersed in a resinous material. The binder materials disclosed in this patent contain a material which is incapable of transporting for any significant distance injected charge carriers generated by the photoconductive zinc oxide particles. Accordingly, as a result the photoconductive particles must be in a substantially contiguous particle to particle contact throughout the layer for the purpose of permitting charge dissipation required for a cyclic operation. Thus, with the uniform dispersion of photoconductive particles described a relatively high volume concentration of photoconductor material, about 50 percent by volume, is usually necessary in order to obtain sufficient photoconductor particle to particle contact for rapid discharge. This high photoconductive loading can result in destroying the physical continuity of the resinous material thus significantly reducing the mechanical properties of the binder layer. Illustrative examples of specific binder materials disclosed in this patent include, for example, polycarbonate resins, polyester resins, polyamide resins, and the like.
There are also known photoreceptor materials comprised of other inorganic or organic materials wherein the charge carrier generation and charge carrier transport functions are accomplished by discrete contiguous layers. Additionally, photoreceptor materials are disclosed in the prior art which include an overcoating layer of an electrically insulating polymeric material and in conjunction with this overcoated type photoreceptor there have been proposed of number of imaging methods. However, the art of xerography continues to advance and more stringent demands need to be met by the copying apparatus in order to increase performance standards, and to obtain higher quality images. The photoconductive imaging member of the present invention represents such an improved member and has other advantages as disclosed hereinafter.
Recently, there has been developed other layered photoresponsive devices including those comprised of generating layers and charge transport layers as disclosed in U.S. Pat. No. 4,265,990, and overcoated photoresponsive materials containing a hole injecting layer, covercoated with a transport layer, followed by an overcoating of a photogenerating layer and a top coating of an insulating organic resin, reference U.S. Pat. No. 4,251,612. Examples of generating layers disclosed in these patents include trigonal selenium and vanadyl phthalocyanine, while examples of transport layers that may be employed are comprised of certain diamines as mentioned herein. The disclosures of each of these patents, namely, U.S. Pat. Nos. 4,265,990 and 4,251,612 are totally incorporated herein by reference.
Many other patents are in existence describing photoresponsive devices including layered devices containing generating substances such as U.S. Pat. No. 3,041,167, which discloses an overcoated imaging member containing a conductive substrate, a photoconductive insulating layer, and an overcoating layer of an electrically insulating polymeric material. This member is utilized in an electrophotographic copying method by, for example, initially charging the member, with an electrostatic charge of a first polarity and imagewise exposing to form an electrostatic latent image which can be subsequently developed to form a visible image. Prior to each succeeding imaging cycle, the imaging member can be charged with an electrostatic charge of a second polarity which is opposite in polarity to the first polarity. Sufficient additional charges of the second polarity are applied so as to create across the member a net electrical field of the second polarity. Simultaneously, mobile charges of the first polarity are created in the photoconductive layer such as by applying an electrical potential to the conductive substrate. The imaging potential which is developed to form the visible image is present across the photoconductive layer and the overcoating layer.
There is also disclosed in Belgium Pat. No. 763,540, an electrophotographic member having at least two electrically operative layers, the first layer comprising a photoconductive layer which is capable of photogenerating charge carriers, and injecting the photogenerated hole into a continuous active layer containing a transport organic material which is substantially non-absorbing in the spectral region of intended use, but which is active in that it allows injection of photogenerating holes from the photoconductive layer causing these holes to be transported through the active layer. The active polymers selected for this device may be mixed with inactive polymers or non-polymeric materials.
In U.S. Pat. No. 3,041,116 there is disclosed a photoconductive material containing a transparent plastic material overcoated on a layer of vitreous selenium which is contained on a recording substrate. Apparently, in operation, the free surface of the transparent plastic is electrostatically charged to a desired polarity, followed by exposing the device to activating radiation, which generates a hole electron pair in the photoconductive layer, causing the electrons to move to the plastic layer, and neutralize the positive charges contained thereon, thus creating an electrostatic image.
Also, there is disclosed in U.S. Pat. Nos. 4,232,102 and 4,233,383 the use of trigonal selenium doped with sodium carbonate, sodium selenite, and trigonal selenium doped with barium carbonate, and barium selenite as photoresponsive imaging members.
Other representative patents disclosing layered photoresponsive devices include U.S. Pat. Nos. 4,115,116, 4,047,949 and 4,081,274.
There is also disclosed in a prior copending application an improved photoresponsive device containing a substrate, a hole blocking layer, an optional adhesive layer, an inorganic photogenerating layer, an organic photoconductive layer, and a top coating of a hole transport layer.
While the above-described photoresponsive devices are suitable for their intended purposes there continues to be a need for improved devices, particularly layered devices, which not only generate acceptable images but which can be repeatedly used in a number of imaging cycles without deterioration thereof from the machine environment or surrounding conditions. Additionally, there continues to be a need for improved layered imaging members wherein the materials selected for the respective layers are substantially inert to users of such devices, while simultaneously functioning as an imaging member. Furthermore, there continues to be a need for imaging members wherein adhesion of the layers such as, for example, the photogenerating layer to the substrate can be accomplished without the need for specific adhesive materials, while simultaneously improving the scratch resistance of the other layers such as the ground plane layer, improving the strength of the binder generating layer and providing a device of improved mechanical strength. Also, there continues to be a need for overcoated photoresponsive devices which are sensitive to a broad range of wavelengths, and more specifically are sensitive to infra-red light and visible light thereby allowing such devices to be useful in a number of imaging and printing systems. Additionally, there continues to be a need for improved photoresponsive devices which can be prepared with a minimum number of processing steps, and wherein the layers are sufficiently adhered to one another to allow the continuous use of such devices in imaging and printing systems with repetitive imaging and printing cycles.